The basic measurements in fuel injection systems are the measurements for determining the amount of fuel to be supplied to the engine. Air/fuel ratios are satisfied by measuring the amount of air intake to the engine and then supplying the proper amount of fuel under control of scheduling tables located in the electronic control unit (ECU).
The most common method to determine the amount of fuel to be injected is to measure the manifold pressure and engine speed and from these measurements determine the amount of fuel. Hot wire anemometers and swirl meters are examples of devices for measuring the amount of air flowing into the engine and with this measurement, the amount of fuel is calculated.
In many devices, a bridge circuit is used in the measuring circuitry. In some, a low resistance is used in the heated leg and a high resistance element is used in the unheated leg. In these situations, both elements are operated at the same voltage requiring the matching of the temperature coefficients of the resistance material with the different values of resistance.
A modification of the above mentioned bridge circuit is to divide the voltage to the leg containing the unheated resistance element and then amplify or multiply the voltage output to cancel the division. This generally requires the use of capacitor compensation to assure stability of the gain stage of the amplifier which in turn slows down the circuit response.
Still other solutions have placed the unheated resistance element in the the feedback circuit of an amplifier in the bridge circuit. This requires a number of interactive function adjustments and a regulated voltage reference.
As noted, in each of these, the measurement circuits require various compensation circuits and matching of temperature coefficients of the different measuring elements. To overcome these difficulties, there is disclosed herein a symmetrical bridge circuit not requiring division and multiplication circuits and capacitor compensation where one leg of the the bridge contains a sensing element heated by the circuit. The other leg of the bridge contains a compensation element unheated by the circuit and is used to compensate for air temperature changes.